Although it can really be abything......i highly doubt it is something like strontium, iodine, or even more commonly tested potassium.....most highly successfully reefers will barely if ever test these. Sometimes the most common canbe overlooked. Are you sure about your salinity, temps, alk, ca, nitrates, possibly even ammonia...? What about ur rodi water? I have mentioned before that i had a crazy high salinity of 1.030+ earlier this year.....i am still bringing it back down....it is at 1.027 now.... this also caused a spike in alk, ca, mag.....i began to see acros rtn or stn eventually until problem was found....the salinity rose so slow and gradual that a lot of my corals were used to it....hell even the fiah did great....now as i am lowering it back down i am also seeing small issues.....i have seen the tissue necrosis on acros stop but just two days ago i have noticed another nice acro colony going stn from the base and looking fadrd in colour in general. Meanwhile other corals are thriving. Its all about stability..... delicate corals will have problems with params like salinity, alk, ca, fluctuating....there are many things that can cause this and the biggest problem is that by affecting one parameter directly, it can then affect other params indirectly....
What i am trying to say is that good change is still change. And change does not equal stability....
I really want all you guys to be successful so my best advice is go back to the basics.....forget about po4 even.... check the major params only and keep them stable. Develope a maintenance routine for your tank and stick to it.
It was a long way to being able to grow acros or even some other corals.....i learned a lot tho....i have watched my tank for so long that i can usually tell if i need to change my di media just by starting to see traces of algae lol...
Oh and i also need to say that by adding things which are good for the tank imo such as pellets, or prodibio (dont hate lol) or gfo or carbon or whatever might be good is still change. Especially pellets!!! Or pellets and prodibio and god knows what else (cough,tony, cough haha) is going to have a major impact on your system. It could take 6-12 months for everythinng to stabilize and thats only if u keep it all STABLE. Probably a bunch of other things i should say too buy man it sucks typing on ur phone.... mind the basics, get a routine, test params as often as u can until the tests show matching results month after month after month after month and then maybe u can cut back to monthly because u have now developed a stable setup....
I will probably think of more to ramble on about later lol...
